Fix browser runner recipe contract

[chubes4]

Summary

• Exposes a reusable browser runner PHP contract on generated browser recipes so partial materializer PHP can inherit the canonical WP Codebox runner prelude instead of depending on JS-side helper stubs.
• Updates the browser runtime to wrap helper-dependent recipe PHP with the contract-provided prelude and fail clearly when helper-dependent partial PHP has no runner contract.
• Adds smoke coverage for the generated runner contract and keeps the lifecycle-hook assertion compatible with the current recipe activation hook shape.

Refs #580.

Verification

• node --check packages/wordpress-plugin/assets/browser-runtime.js
• npm run browser-runtime-operation-smoke
• php tests/smoke-wordpress-plugin.php
• Studio Web live eval run live-2026-06-04T15-38-38-258Z wrote /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/studio-web-headless-eval-fast/evidence.json

Eval notes

• The eval no longer shows the /internal/eval.php missing wp_codebox_browser_* helper/global fatal cascade.
• Browser evidence shows 10 successful /wp-codebox/v1/browser-provider-request responses, so the run reached live provider execution.
• The remaining failure is later downstream Studio Web materialization metadata: Website materialization import_result missing canonical theme_slug; has_artifact_bundle=false in the eval evidence.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Drafting the browser runner contract refactor, smoke coverage, validation runs, and PR evidence summary; Chris remains responsible for review and merge.